Synchronization device for output stages, particularly for electronic memories

Abstract: A synchronization device for output stages, particularly for electronic memories, whose particularity is that it comprises: first logic comparison means which are suitable to check the state of an enable signal of the electronic memory and an enable signal of the output stage of the electronic memory and are suitable to generate a first confirmation signal; flip-flop means which are suitable to be enabled by a data load signal and disabled by the enable signal of the electronic memory and are suitable to generate a second confirmation signal when they are enabled; and second logic comparison means which are suitable to check the first and second confirmation signals so as to generate an actual signal for enabling the output stage of the electronic memory.

Description: The present invention relates to a synchronization device for output stages, particularly for electronic memories. It is known that there are two fundamental control pins in electronic memories. The first one, CEN (inverted chip enable), is the pin that enables the memory. The second one, OEN (inverted output enable), is the pin that enables the output stage of the memory. Figure 1 is a block diagram of a possible conventional memory, wherein the memory circuit is designated by the reference numeral 1. The memory circuit sends the data to a latch circuit 2, which stores the data before sending them to the output stage 3. The memory circuit 1 is enabled by the CEN signal, that is to say, the circuit is enabled when said signal is equal to "0". The CEN signal and the OEN signal constitute the inputs of a NOR gate 4 which enables the output stage 3 if both input signals are "0". Accordingly, if the OEN signal is at logic level "1", the output stage 3 is set to a high impedance, and this allows the external logics to communicate with the interior of the memory. If instead the OEN signal is set to logic level "0", the output stage 3 is enabled to send out the data produced within the memory. This second operating mode is characterized by a response time that is faster than the corresponding time for the propagation of the data in the memory circuit 1 required to fully read the CEN signal. This entails that the output stage 3 can switch randomly or on the basis of data which had been read earlier and are therefore in any case obsolete. It is known that switching on the outputs produces severe noise on the internal circuits of the memories and in general slows down the read times; the more this event occurs asynchronously and the higher the number of outputs, the slower the read time. Accordingly, it is desirable to avoid false switchings in order to achieve best performance. A LOAD signal is generated within the memory circuit, which activates the latch circuit 2 when the data from the memory circuit 1 reach it. However, this has no effect on the output stage 3, which might include false or useless data. US-A-4,858,197 discloses a synchronization device which however, does not include forced activation means and first comparison means as recited in claim 1. A principal aim of the present invention is to provide a synchronization device for output stages, particularly for electronic memories, which is capable of synchronizing the output stages of the memories so as to avoid false switching. Within the scope of this aim, an object of the present invention is to provide a synchronization device for output stages, particularly for electronic memories, which allows to optimize propagation times and allows to have only reliable data on the outputs. Another object of the present invention is to provide a synchronization device for output stages, particularly for electronic memories, which is capable of handling contingent situations in an electronic memory. Another object of the present invention is to provide a device which is highly reliable, relatively easy to manufacture, and at competitive costs. This aim, these objects, and others which will become apparent hereinafter are achieved by a synchronization device for output stages, particularly for electronic memories, as defined in claim 1. The characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a block diagram of a conventional memory; figure 2 is a block diagram of a further conventional memory; figure 3 is a block diagram of a still further conventional memory; figure 4 is a block diagram of an embodiment of the device according to the present invention; figure 5 is a view of a possible embodiment of the device according to the present invention, shown in figure 4; figure 6 plots the waveforms related to the operation of the device according to the present invention; figure 7 is a view of another embodiment of the device according to the present invention. With reference to figure 2, the synchronization device according to a further conventional memory will be described. The device comprises first logic comparison means 5 which are suitable to determine whether the two inputs CEN and OEN are at the enabling logic level that is suitable to activate the memory device and its output stage 3. Normally, both signals CEN and OEN should be at logic level "0" to activate the output stage 3. The memory enable signal CEN is also sent to flip-flop means 6 together with the data load signal LOAD. The signal CEN is sent to the disable input, whereas the signal LOAD is sent to the enable input of the flip-flop means 6. The outputs of the first logic comparison means 5 and of the flip-flop means 6 are sent to second logic comparison means 7 which are suitable to generate the actual signal OE that enables the output stage 3 of the memory. Figure 3 illustrates a possible arrangement not covered by the invention with logic gates of the device of figure 2. The first logic comparison means are constituted by a first NOR gate 8, whereas the flip-flop means are constituted by a second NOR gate 9 and by a third NOR gate 10. The output of the third NOR gate 10 is connected to an input of the second NOR gate 9 and vice versa. The enable signal CEN is applied to the remaining input of the second NOR gate 9, and the load signal LOAD is applied to the remaining input of the third NOR gate 10. The output of the second NOR gate 9 is the output of the flip-flop means. The outputs of the first logic comparison means and of the flip-flop means, as mentioned, are sent to the second logic comparison means, which are constituted by a NAND gate 11 and by an inverting gate 12. The second logic comparison means generate the actual signal OE that enables the output stage 3. The flip-flop means 6 are meant to control the enabling of the actual signal OE that enables the output stage. The flip-flop means 6 are in fact reset (disabled) by the memory enable signal CEN so as to prevent the first enabling on the part of the enable signal of the output stage OEN. The signal LOAD, which indicates that the data has propagated in the memory, enables the flip-flop means 6 for the signal OEN. The operation of the device according to the prior art is as follows. Every time the signal CEN is set to logic level "1", the flip-flop means 6 are reset by the signal CEN itself and the circuit located downstream of said means is disabled. This reset condition persists until CEN is equal to logic level "1". The output stage enable signal OEN cannot produce any effect in this configuration, since the first logic comparison means 5, that is to say the NOR gate 8, are controlled by the forcing condition of the signal CEN. The data load step, represented by the signal LOAD, is assuredly at logic level "0". As soon as the memory enable signal CEN is set to logic level "0", the disabled condition of the flip-flop means 6 ceases and said flip-flop means 6 can be enabled by the signal LOAD (producing a "1" on the output of the second NOR gate 9), which assuredly occurs after a wait time that is equal to the propagation time of a normal data reading operation. If in the meantime the signal OEN is set to logic level "0", it can propagate, through the first NOR gate 8, the first confirmation signal to the second logic comparison means 7, that is to say, to the NAND gate 11 and to the inverter 12; this, however, produces no effect downstream, since it is necessary to wait for the second confirmation signal that arrives from the flip-flop means 6. Said second enabling occurs only at the end of the propagation of the data in the memory, that is to say, as mentioned, when the signal LOAD is enabled. If instead the signal OEN continues to be kept at logic level "1", then even resetting of the flip-flop means 6 does not enable the actual output stage enable signal OE. In these conditions, the first switching of OEN to logic level "0" produces the immediate effect of propagation downstream of the NAND gate 11, consequently enabling the actual output stage enable signal OE and therefore the output stage 3. Figure 6 illustrates in the interval T9-T10 the typical waveforms of the device according to the present invention. Initially, at the time T 0 , the signal CEN and the signal OEN are at logic level "1" and accordingly the device is not enabled. In fact, during the interval T 1 -T 0 the signal OEN varies from "1" to "0" without any effect, since CEN is always at level "1". At the time T 1 , the signals CEN and OEN reach logic level "0" and the device is enabled. However, since a signal LOAD is missing, the actual output stage enable signal OE remains unchanged. At the time T 2 , the signal LOAD is sent to the flip-flop means 6, and since the signals CEN and OEN are at level "0", the actual signal OE reaches logic level "1" and thus enables the output stage 3. At the time T 3 , the output stage enable signal OEN becomes "1" and therefore the actual signal OE also becomes "0". At the time T 4 , the signal OEN again reaches "0" and the actual signal OE returns to the enabled state ("1"), since the flip-flop means 6 have not been reset by the signal CEN. At the time T 5 , the signal CEN reaches logic level "1" and thus disables the actual signal OE even if the signal OEN has remained at logic level "0". At the time T 6 , the signal CEN again reaches logic level "0", and thus the device is enabled. When the signal CEN is altered, the flip-flop means 6 are reset and a further enabling of the actual signal OE must wait for a new signal LOAD. At the time T 7 , after the data propagation time T 7 -T 6 , the flip-flop means 6 are enabled by a signal LOAD. A logic level "1" is present at the output of the first logic comparison means 5, since CEN and OEN are at logic level "0", and therefore the first confirmation signal occurs. A logic level "1" is present at the output of the flip-flop means and constitutes the second confirmation signal, which together with the first enabling of the first logic comparison means 5 activates the second logic comparison means 7 (the NAND gate 11 and the inverter 12), which generate an actual signal OE at logic level "1". At the time T 8 , the actual signal is disabled, since the output stage enable signal OEN returns to logic level "1" and the first logic comparison means 5 do not produce the first confirmation signal. An embodiment of the present invention is shown in figure 4. The device is identical to the one of figure 2, except that forced activation means 13 are interposed between the flip-flop means 6 and the second logic comparison means 7. With reference to figure 5, the forced activation means can be constituted by an OR gate 14 which, in the presence of a forced signal OE forced , generates the second confirmation signal and is thus capable, together with the first logic comparison means 5 (the NOR gate 8), of activating the second logic comparison means (the NAND gate 11 and the inverter 12) to generate an actual output stage enable signal OE. This is shown in figure 6 in the interval T 10 -T 9 , where without the presence of a signal LOAD and with the signals CEN and OEN at logic level "0" the signal OE forced is able to force an actual enabling signal OE. Such a condition is provided for all those contingent situations in which internal propagation times are negligible and therefore require immediate reading. These contingent situations may be readings of internal parts such as byte identifiers or registers, testing activities, etcetera. Figure 7 illustrates another embodiment of the device according to the present invention. Like the embodiments described above, the device is provided with first logic comparison means, constituted by a NOR gate 8, to which the signals CEN and OEN are sent. The device is furthermore provided with flip-flop means which are constituted by two NAND gates 15 and 16. Since in this case NAND gates are used instead of NOR gates as before, the inputs of the flip-flop means are an inverted LOAD signal LOADn and an enable signal CE (which is not inverted). The inverted CE signal (CEN) used by the first logic comparison means can be generated by means of an inverter 17. The first logic comparison means and the flip-flop means are connected to second and third logic comparison means. The second logic comparison means are constituted, as before, by a NAND gate 11 and by an inverter 12. Forced activation means, advantageously constituted by an OR gate 14, are furthermore associated with the second logic comparison means. The third logic comparison means are constituted by a three-input NAND gate 11a and by an inverter 12a which are connected in series. In this case, too, forced activation means, constituted by the OR gate 14a, are associated with the third logic comparison means. The output of the second logic comparison means is the actual output stage enable signal OE, whereas the output of the third logic comparison means is an enable signal OE high for the output stage of the most significant bits. In this manner it is possible to enable the outputs in packets; that is to say, it is possible to enable the output stages in groups. The particularity of this embodiment resides in the fact that the signal sent to one of the three inputs of the NAND gate 11a is a signal WORD mode for enabling the third logic comparison means. This signal is suitable to enable or disable the third logic comparison means so as to enable or disable the outputs that it controls. Multiple packets of output stages are controlled in this manner. The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. Thus, for example, it is possible to provide the devices according to the present invention with combinations of logic gates that are different from those described. All the details may furthermore be replaced with other technically equivalent ones , as for as they fall within the scope of the appended claims. In practice, the materials employed, as well as the shapes and dimensions, may be any according to the requirements without thereby abandoning the scope of the protection of the appended claims. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A synchronization device for output stages, particularly for electronic memories, comprising: first logic comparison means (5;8) which are suitable to check the state of an enable signal (CEN;CE) of the electronic memory and an enable signal (OEN) of the output stage (3) of said electronic memory, said first logic comparison means (5;8) being suitable to generate a first confirmation signal; flip-flop means (6;15,16) which are suitable to be enabled by a data load signal (LOAD;LOADn) and disabled by said electronic memory enable signal (CEN;CE) and are suitable to generate a second confirmation signal when they are enabled; second logic comparison means (7;11,12), connected to the outputs of said first logic comparison means (5;8)) and of said flip-flop means (6;15,16) and which are suitable to check said first and second confirmation signals so as to generate an actual signal (OE) for enabling said output stage (3) of said electronic memory; the device further comprising forced activation means (13) which are interposed between said flip-flop means (6;15,16) and said second logic comparison means (7) and are suitable to generate said second confirmation signal when a forced activation signal (OE forced ) is received. The device according to claim 1, characterized in that it comprises third logic comparison means (11a,12a) which are parallel-connected to said second logic comparison means (11,12) and have an input to which a signal for enabling said third logic comparison means is applied. The device according to one or more of the preceding claims, characterized in that said forced activation means (14a) are interposed between said flip-flop means (15,16) and said third logic comparison means (11a,12a). The device according to claim 1, characterized in that said first logic comparison means (8) are constituted by a NOR gate. The device according to claim 1, characterized in that said flip-flop means (6) are constituted by a first NOR gate (9;15) and by a second NOR gate (10;16), the output of said first NOR gate being connected to one of the inputs of said second NOR gate and the output of said second NOR gate being connected to one of the inputs of said first NOR gate. The device according to claim 1, characterized in that said second logic comparison means (7) are constituted by a NAND gate (11) which is series-connected to an inverter (12). The device according to claim 1, characterized in that said forced activation means (13) are constituted by an OR gate (14;14a).

Inventors: PASCUCCI LUIGI, PASCUCCI, LUIGI
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G11C7
IPC codes: G11C11, H04L7, G11C7